With regard to area behind tank - have you considered the DIY, 2-part undercoating paint. I believe marketed for beds, but I used on floor and interior firewall area as a noise reducer. While not original, I was not offended by the look on my floor and wished I went up the back of the cab.
~ HB 1966 Chevrolet K-10 | Ghost: formerly Flappy Fenders | In the Stovebolt Gallery 1962 Chevrolet C10 1962 Suburban